What is the cause for stills disease?

No simple cause. "still's" is a synonym in many places for juvenile rheumatoid arthritis which is at least three diseases, all of unknown etiology that represent disturbances in the immune system. Mimics include lyme disease ; parvo b19. You're unlikely to find the etiology for a particular case of true still's; rarely an elimination diet might reveal a simple food allergy but this is the exception.
Still's disease. Still's disease is an auto-immune disease, such as lupus and dermato-myositis. At time it is associated with specific hla antigens. It is a syndrome of arthritis, high fever associated with elevated acute phase reactants. The diagnosis is one of exclusions specally sepsis.